Page 168 - Handbook of Battery Materials

4.3 Physical Properties and Chemical Composition of EMD 137

4.3
Physical Properties and Chemical Composition of EMD

Table 4.6 shows the physical properties and chemical composition of typical battery
active EMDs, and Table 4.7 shows a typical chemical analysis of EMD. Figure 4.14
shows a schematic model of an EMD particle showing three types of pores.
Figure 4.15 shows the calculated surface area of nonporous solid cubes having a
−1
specific gravity (SG) of 1.0, 4.0, and 5.0 g mL . As the measured SG of EMD is
−1
2
4.3 g mL −1 and the surface area is 25–35 m g , the superficial surface area is less
2
−1
than 0.1 m g . Therefore, most of the electrochemical reaction takes place on the
pore wall of the fine pores and cavities since the pore diameter is 50–100 ˚ A.
The ideal battery material should be highly porous, but should have a high
density in order to pack as much as possible in to the limited space of the cell. EMD
is almost the ideal MnO 2 since it is dense and has fine pores (actually cavities).
4.3.1
Cross-Section of the Pores

Based on the gas adsorption behavior, Kozawa and Yamashita proposed a hypothesis
[19, 20] that the cross-section of the fine pores of EMD is a cavity shape as shown
in Figure 4.16. The cross-section of the pore by computer calculation is a circle
(Figure 4.16a). Nobody knows the real shape of the cross-section as yet. Kozawa’s
belief in the cavity shape (Figure 4.16b) is based on the results of experiments
involving the oxygen adsorbed and desorbed from the pore walls [19].

Table 4.6 Physical properties and chemical composition of EMD.

Physical properties
Electrical resistivity a 50–100 cm
2
BET surface area 40–50 m g –1
Pore diameter 40–60 ˚ A
Pore volume (<150 ˚ A pores) 0.032–0.035 mL g −1
Density
True density 4.0–4.3 g mL −1
Apparent density 2.2–2.3 g mL −1
Average particle size 10–45 in.
Chemical composition
MnO 2 92%
H 2 O 2–4%
Mn 2 O 4 and MnO 3–4%
so 2− ∼1%
4
Metallic impurity Very low

a
Measured with powder sample in tablet from under high pressure.